When metal strip is passed between the work rolls of a cold rolling mill, it is essential that the strip being delivered by the mill has the same profile as the strip entering the mill, the strip profile being defined as the variation in thickness across the width of the strip, expressed as a percentage of the thickness at the middle of the strip. This can only be achieved by giving every element of strip across the width of the strip the same percentage reduction, and the same elongation as every other element. To satisfy this condition, the profile of the roll gap during rolling (i.e. the roll gap profile when the rolling mill structure is deflected under the action of the roll separating forces which occur during rolling) must be identical to the desired profile of the strip being delivered, this profile being identical to that of the incoming strip.
Virtually all rolling mills have the characteristic that, under the action of the roll separating force, the rolls deflect away from the strip a greater amount at the middle of the strip than at the strip edges. As a result, when rolling strip having a uniform profile with cylindrical mill rolls, the profile of the roll gap during rolling, and hence of the strip being delivered, is convex (as shown in FIG. 1). In this case, the percentage reduction and elongation are greater at the strip edges, and the issuing strip has the flatness defect known as "long edge" or "wavy edge".
To overcome this effect, as is well known in the art, some or all of the rolls in a rolling mill are provided with a convex profile or "crown". This crown is usually produced by grinding the roll(s) in a roll grinding machine equipped with a crowning attachment. Such crowning attachments usually enable profiles only of parabolic or similar forms to be achieved. For small diameter Sendzimir mill work rolls, the crown can also be produced by bending the work roll in the grinding machine (using a system of steady rests) and grinding the bent roll as it rotates in the machine.
As shown in FIGS. 2a and 2b, when either one or two crowned rolls are used in the mill, it is possible to achieve uniform profile of the roll gap, and hence of the strip being delivered. In this case, if the profile of the incoming strip is uniform, the flatness of the issuing strip will be perfect.
In the case of FIG. 2b where one crowned and one uncrowned roll are used, it can be seen that the strip must flex laterally, but apart from this flexure, the strip has the same profile as the strip in FIG. 2a, where two crowned rolls are used. Since the strip is relatively flexible in the transverse direction, this causes no problems, and since it is often very convenient to grind a crown in one work roll only, rolling with one crowned and one uncrowned roll is very well known in the art.
When rolling with high separating forces large crowns are needed. When rolling with lower separating forces smaller crowns are needed, since the roll deflection is smaller at the smaller separating force. In theory, any one particular crown is only correct at one value of separating force, while in practice, it would be satisfactory for a small range of separating force. To give more flexibility, and avoid the need for frequent roll changes, when the roll separating force levels change (as, for example, when changing from rolling a thick, hard alloy to rolling a thin soft alloy), many rolling mills are provided with profiling controls. These fall into four main classes. These are roll bending controls (class 1), axial shifting controls (class 2), roll crossing controls (class 3) and thermal profiling controls (class 4).
It should be noted that all of these controls have the ability to control the strip thickness at the edge of the strip relative to the thickness at the middle of the strip. However, with the possible exception of thermal profiling, none of these can control the strip thickness at the quarter bands (i.e. those regions of the strip lying anywhere between the strip middle and the strip edges) independently of the thickness at the edge of the strip. For example, with roll bending controls, operating the controls to bend the roll ends away from the strip, as shown in FIG. 3, makes the strip edges thicker relative to the middle, but also makes the quarter bands thicker. Operating the controls to bend the roll ends towards the strip, as shown in FIG. 4, makes the strip edges thinner, but also makes the quarter bands thinner relative to the middle of the strip.
One of the more common types of flatness defect appearing on strip rolled by cold rolling mills is quarter buckle, a condition where the strip thickness at the quarter bands has been reduced too much, and the material at the quarter bands has been elongated too much relative to the material at middle and edges of the strip. Usually in strip having a flatness defect of the quarter buckle variety, elongation at the strip middle and the edges is similar, so neither center buckles (caused by excessive elongation at strip middle) or edge waves (caused by excessive elongation at strip edges) can be seen on the strip. Clearly strip having a flatness defect of the quarter buckle variety cannot have its flatness corrected by the prior art controls of classes 1, 2 and 3. Some correction of quarter buckle defect can be achieved on four high rolling mills rolling soft material such as aluminum using thermal profiling by means of modulation of coolant spray distribution, a class 4 profiling control, but such methods are ineffective or have very limited range when rolling harder metals, or when using cluster mills.
One object of the present invention is to provide means and a method for cold rolling metal strip which will inherently produce strip with much less quarter-buckle than prior art methods. The invention contemplates novel profiles for one, some or all of the rolls in a cold rolling mill.
A further object of the invention is to provide means and a method for cold rolling metal strip which takes into account the actual profile of strip being supplied to the mill, and determines the optimum profile or profiles of rolls in the mill accordingly.
Still another object of the invention is to provide a method of controlling the profile of the gap between the work rolls, and thus the profile of the issuing strip, so that the thickness of the strip in the area of the quarter bands can be controlled independently of the thickness of the strip at middle and edges.
It is known in the art that strip rolled on a hot rolling mill is characterized by a thinning of the strip in the area of the strip edges. This thinning is known as "feather", "edge drop" or as "edge droop" and can be expressed as a percentage of the gauge at the middle of the strip. It is also known that strip produced on a cold rolling mill, at heavy gauges, also develops edge drop as rolling proceeds. For example, copper and brass, which are frequently continuously cast at about 3/4 inch thickness and about 25 inches wide, and are subsequently machined to a virtually uniform profile at about a 5/8 inch thickness, will commonly develop an edge drop of about 3% when cold rolled down to 1/8 inch thickness on a cold breakdown mill.
A typical cross section of a strip 20 produced by a hot rolling mill or a heavy gauge breakdown mill is shown in FIG. 5, with the strip profile indicated at 21. It can be seen from FIG. 5 that the strip thickness over the central 75% to 85% of the width exhibits a smooth and minor drop-off from the middle to the edges of this zone, i.e. it is very slightly crowned. For the last 5% to 10% of the strip width, the thickness drops off very rapidly towards each edge.
It is believed that the edge drop phenomenon is due to lateral sidespread of the strip in the hot mill or cold breakdown mill roll bite. For the central 75% to 85% or so of the width (depending upon thickness and width of the strip) the material in the roll bite is constrained by friction between the strip and the rolls, to elongate in the direction of rolling only. The material in the roll bite close to the strip edges, however, is free to spread sideways (i.e. in the direction of the roll axes) to some extent, as well as elongating in direction of rolling. The result of this freedom is that (a) the strip is thinner at the edges, because each element of strip close to the edges is made wider, as well as longer, as it passes through the roll bite and (b) the strip at the edges is a little shorter than the strip at the middle. It is possible to show the widening effect by measuring strip width before and after hot rolling. The edge shortening effect can be demonstrated by passing the strip under tension over a deflector roll. It will be seen that the strip edges "take a short cut" i.e. they hug the roll tightly and will even cut into the roll, whereas the middle of the strip (depending upon strip thickness and tension) may not even touch the roll.
FIG. 6 shows what happens when attempts are made to roll the strip 20 with a rolling mill having prior art roll profiles, of simple crowned form 22, as shown in FIG. 5. It is assumed that the mill profile is adjusted by selecting the correct theoretical crown, so that the elongation at the strip edge is the same as that at the strip middle. Because the strip is thinner at the edges, the roll gap must be given a convex profile 22a, as shown in FIG. 6, the curve 22a representing the deflected form of the roll profile 22 of FIG. 5. It will be understood that in FIG. 6 the profile 22a of the rolls in deflected condition (by virtue of the roll separating forces), the roll gap profile, and the profile of the exiting strip 20 are all the same.
By comparing FIG. 6, which shows the roll gap or strip profile 22a of the strip leaving the rolling mill, with FIG. 5, which shows the profile 21 of the strip entering the rolling mill, it can be seen that the profiles are different.
Note that, in the drawings, profile 22a of FIGS. 6 and 7 is the deflected form of the profile 22 of the rolls of FIG. 5. FIG. 7 shows the desired strip exit profile 21, (to achieve good strip flatness) which is identical in form to the incoming strip profile 21 of FIG. 5, superimposed upon the actual profile 22a of strip produced by the mill with prior art roll profiles. It can be seen from FIG. 7 that the strip in the area of the quarter bands has been made thinner than the ideal thickness indicated by the desired strip exit profile. The areas 23 of the strip section
represent the difference between the ideal strip profile 21 and the actual strip profile 22a, and are a measure of the profile error. This "over-rolling" in the area of the quarter bands results in higher elongation in the area of the quarter bands than in the rest of the strip. This, in turn, causes the strip in the area of the quarter bands to buckle and produce the flatness defect known as "quarter buckle".
To achieve the desired strip exit profile 21, prior art rolls would have to bend very sharply close to the strip edges, as shown in FIG. 7. In practice they do not bend, firstly because of their flexural stiffness and secondly because there are no forces developing which could cause a sharp bend at the ends without causing too much bend at the middle of the rolls. Thus the "over-rolling" of the quarter bands is very commonly seen on all types of cold rolling mills, including those with large (and therefore flexurally stiff) work rolls and those with small (and therefore flexurally soft) work rolls.